CMake: Fix tst_qaddpreroutine to work with static builds

DEFAULT_IF needs to be true so that in a static build the
built plugin is automatically linked into the test executable
using the special static plugin per-repo behavior in
qt_internal_add_executable.

The QtPostProcess routines are not executed for this test project
because we don't use qt_build_repo. This means that no
QtFooPluginCMakeConfig.cmake file is generated and thus there's no
point in using qt6_import_plugins because the pulic plugin genexes
won't know about this target anyway.

Explicitly set the CLASS_NAME so that the name expected by the
Q_IMPORT_PLUGIN macro matches the name of the plugin instance that moc
generates in QT_MOC_EXPORT_PLUGIN.

Amends 22e967c304

Task-number: QTBUG-87580
Task-number: QTBUG-90341
Pick-to: 6.1 6.0
Change-Id: I5ef361e7e2cebc46b35310c679f15c84cd61b4a5
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

tests/auto/cmake/tst_qaddpreroutine/CMakeLists.txt

@@ -24,8 +24,9 @@ find_package(Qt6 COMPONENTS Gui Test CONFIG REQUIRED)
 qt_internal_add_plugin(QTBUG_90341ThemePlugin
     OUTPUT_NAME QTBUG_90341
     OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}"
+    CLASS_NAME ThemePlugin
     TYPE platformthemes
-    DEFAULT_IF FALSE
+    DEFAULT_IF TRUE
     SOURCES
         plugin.cpp
     SKIP_INSTALL
@@ -42,7 +43,6 @@ qt_internal_add_test(tst_qaddpreroutine
         Qt::Gui
 )
 
-qt6_import_plugins(tst_qaddpreroutine INCLUDE QTBUG_90341)
 
 target_compile_definitions(tst_qaddpreroutine
     PRIVATE QT_QPA_PLATFORM_PLUGIN_PATH=\"${CMAKE_BINARY_DIR}\")